If you're using Grizzly's HttpServer, enabling monitoring is very simple:

public static void main(String[] args) {

        HttpServer gws = new HttpServer();
        HttpServer gws1 = new HttpServer();
        NetworkListener listener1 = new NetworkListener("listener1", "localhost", 19080);
        NetworkListener listener2 = new NetworkListener("listener2", "localhost", 19081);
        gws.addListener(listener1);
        gws1.addListener(listener2);

        try {
            gws.start();
            gws1.start();
            gws.getServerConfiguration().setJmxEnabled(true);
            gws1.getServerConfiguration().setJmxEnabled(true);
            assertTrue(true);
        } catch (IOException ioe) {
            ioe.printStackTrace();
            System.exit(1);
        } finally {
            try {
                gws.stop();
                gws1.stop();
            } catch (IOException ignored) {}
        }
}

Notice that enabling or disabling JMX support is dynamic - no need to restart the server instances.